1. Field of Invention
This invention pertains to beamformers which are controlled by means of time delay and tunable lasers.
2. Description of Related Art
Future multi-dimensional array antenna systems will most likely require true time-delay beam steering instead of phase-shifted beam steering since true time-delay beamformers have the ability to steer wide bandwidth signals without beam squint, i.e., frequency dependent steering. Current all-microwave based true time-delay beamformers suffer from severe drawbacks and are currently impractical. Using optical techniques to form 2-D beamformers has been effective at producing true time-delay systems, however, there have been many problems with past designs, including system complexity, optical power loss, component reproducibility, signal stability, and most importantly system cost. These constraints have effectively prevented optical true time-delay beamformers from being implemented in practical wideband antenna arrays having a large number of elements.
Several architectures have been suggested to reduce the cost and complexity issues associated with 2-D and other beamformers, which are summarized as follows:
(A) First are approaches that use switched optical time delay units, but reduce the number of time delay units required via wavelength division multiplexing at each individual antenna element. These approaches still require a large number of time delay units for 2-D and other steering, with some implementations trading off the number of optical time delay units for other inherent system wide problems such as a large number of matched, stable, narrow linewidth, precisely tunable lasers.
(B) A second approach aims at reducing the complexity and cost of the individual time delay units. One design is to use simple dispersive fiber links to implement a dispersive-prism for the time delays required, with a single tunable optical source per steering dimension. For 2-D control, this approach requires N time delay units for the first steering axis, and Nxc3x97N or N2 time delay units for the second steering axis, for a total of N+N2 time delay units. The approach of cascading two sets of independent optical dispersive prisms, increases system cost, increases RF matching errors between individual elements, and reduces dynamic range. Two independently controlled wavelength tunable lasers or two sets of single wavelength switched laser banks also significantly increase system cost. Furthermore, this approach suffers from increased power loss due to two sets of inefficient RF-optical-RF conversions.
(C) The third approach uses a 2-D parallel free-space delay line switching architecture relying on cascading bits selected with spatial light modulators. This approach suffers from bulk optic problems of simultaneous alignment of hundreds of optical paths through multiple cascaded delays, long-term stability problems, optical diffraction over long distances, optical power matching between all channels, and free space coupling problems to wide bandwidth photodetectors.
A beamformer of a phase or time steered radar system is that component which electrically generates the appropriate amount of phase or time delay required to electronically control the propagation direction of the RF radiation which is emitted from the array antenna.
An array antenna apparatus for producing a time-delay beamformed RF signal includes a laser, a modulator for RF modulating the laser optical signal, a first plurality of optical paths coupling the modulator output to a dispersive prism, another plurality of optical paths coupling the dispersive prism outputs to a corresponding plurality of time delay trimmers, an attenuator coupled to each time delay trimmer, and an optical switching matrix to which the time delay trimmers outputs are input. The inputs to the optical switching matrix accordingly are time and amplitude adjusted to remove front end errors prior to rerouting. This is repeated in the back end of the apparatus, in which the switching matrix outputs are each applied to a corresponding attenuator, the outputs of which are then each applied to a photodetector to generate a plurality of replica RF signals that are each applied to a time shifter, thereby producing an error-corrected output that is then applied to an antenna array. The output of the antenna array is a time-delay beamformed RF signal having a 2-D beamsteering capability.
It is an object of this invention to reduce the number of time delay units in forming a photonic beamformer.
It is another object of this invention to reduce the number of steering lasers in forming the architecture of a beamformer.
It is another object of this invention to reduce the complexity of producing an optical beamformer.
It is another object of this invention to minimize the time delay error associated with a reconfigurable optical switching matrix.
It is another object of this invention to displace radar beamformers operating on microwave phase shifter technology with beamformers characterized by a fiber optic time delay units.
Another object of this invention is reduction of electrical power by a factor of about 10-100, which is no longer required as there is less optical and RF loss in this system design.
Another object of this invention is making the beamformer smaller and lighter weight, which is of paramount importance to both the aerospace and satellite applications.
These and other objects can be achieved by a beamformer system, and method for its operation, which system is designed to transmit and steer a radiated RF beam from an antenna array wherein a continuous wave optical carrier is provided by a laser following which optical power is amplified, modulated, again amplified, divided into several optical paths, fed into a multi-channel dispersive prism which includes a time-delay gradient, the optical paths are further divided and are phase and amplitude trimmed, rerouted through an optical switch, further phase and amplitude adjusted, converted from light to RF power and then presented to an antenna array for emission.
Additional features and advantages of the present invention will be set forth in, or be apparent from, the detailed description of preferred embodiments which follows.